The invention relates to a circuit comprising an amplifier comprising an input for receiving an input signal from an input of the circuit, and an output for delivering an output signal to an output of the circuit; and means for reducing the DC-offset and the noise produced by the amplifier.
Such a circuit is known from the general state of the art as shown in FIG. 1. The known circuit comprises an amplifier AMP having a differential input 5;6 comprising an input node 5 and an input node 6, and a differential output 7;8 comprising an output node 7 and an output node 8. The circuit further comprises an input chopper CHP.sub.i arranged in cascade in between a differential input 1;2 of the circuit and the differential input 5;6 of the amplifier AMP, and an output chopper CHP.sub.o synchronised with the input chopper CHP.sub.i, which output chopper CHP.sub.o is arranged in cascade in between the differential output 7;8 of the amplifier AMP and a differential output 3;4 of the circuit. The operation of the known circuit is as follows. An input signal V.sub.in is supplied to the differential input 1;2. In response to the input signal V.sub.in an output signal V.sub.out is delivered to the differential output 3;4. The input chopper CHP.sub.i is arranged for alternately coupling either input terminals 1 and 2 of the differential input 1;2 to respectively input nodes 5 and 6 of the differential input 5;6 during a first phase PHi.sub.1, or input terminals 1 and 2 of the differential input 1;2 to respectively input nodes 6 and 5 of the differential input 5;6 during a second phase PHi.sub.2. The output chopper CHP.sub.o is arranged for alternately coupling either output nodes 7 and 8 of the differential output 7;8 to respectively output terminals 3 and 4 of the differential output 3;4 during a first phase Phi.sub.1, or output nodes 7 and 8 of the differential output 7;8 to respectively output terminals 4 and 3 of the differential output 3;4 during a second phase PHi.sub.2. The input signal V.sub.in is converted to the output signal V.sub.out in virtually the same manner as in the case that the input chopper CHP.sub.i and the output chopper CHP.sub.o would not have been present. However by the presence of the input chopper CHP.sub.i and the output chopper CHP.sub.o the offset produced by the amplifier AMP, which offset is denoted as V.sub.os in FIG. 1, is eliminated. Also the noise produced by the amplifier AMP is significantly reduced for frequencies lower than the switching frequency of the switches in the input chopper CHP.sub.i and the output chopper CHP.sub.o. For the purpose of reducing the noise produced by the amplifier AMP the aforementioned switching frequency must be as high as possible.
A problem of the known circuit is that for a high switching frequency the so-called charge injection caused by the switches of the input chopper CHP.sub.i and the output chopper CHP.sub.o causes a DC-offset in the output signal V.sub.out. Thus with the known circuit an optimal DC-offset reduction can only be accomplished if the switching frequency is low. In that case the noise reduction is not optimal. On the other hand an optimal noise reduction is accomplished if the switching frequency is high. In that case however the DC-offset reduction is not optimal. Thus, with the known circuit, an optimal DC-offset reduction and an optimal noise reduction can not be accomplished at the same time.